one-liners for the remaining top-50 dart:core classes

sdk/lib/core/num.dart

Index: sdk/lib/core/num.dart
diff --git a/sdk/lib/core/num.dart b/sdk/lib/core/num.dart
index b35a5ad5008902fc37717d11a5b37e52cccd899b..e5ac21adf8fb360ad4595007da06d7f5724ccd14 100644
--- a/sdk/lib/core/num.dart
+++ b/sdk/lib/core/num.dart
@@ -5,9 +5,9 @@
 part of dart.core;
 
 /**
- * All numbers in dart are instances of [num].
+ * An integer or floating-point number.
  *
- * It is a compile-time error for any type other than the types int and double
+ * It is a compile-time error for any type other than [int] or [double]
  * to attempt to extend or implement num.
  */
 abstract class num implements Comparable<num> {